A paradigm for control of insulin secretion is that glucose metabolism elevates cytoplasmic [ATP]/[ADP] in β cells, closing K_ATP channels and causing depolarization, Ca²⁺ entry, and insulin release. Decreased responsiveness of K_ATP channels to elevated [ATP]/[ADP] should therefore lead to decreased insulin secretion and diabetes. To test this critical prediction, we generated transgenic mice expressing β cell K_ATP channels with reduced ATP sensitivity. Animals develop severe hyperglycemia, hypoinsulinemia, and ketoacidosis within 2 days and typically die within 5. Nevertheless, islet morphology, insulin localization, and α and β cell distributions were normal (before day 3), pointing to reduced insulin secretion as causal. The data indicate that normal K_ATP channel activity is critical for maintenance of euglycemia and that overactivity can cause diabetes by inhibiting insulin secretion.